Earthquake downgraded to 3.8

SYCAMORE – An earthquake hit northern Illinois just before 4 a.m. Wednesday, waking up residents throughout the region, including in McHenry County.

The 3.8-magnitude earthquake happened at 3:59:34 a.m., according to the U.S. Geological Survey, and was about 3.1 miles underground.The quake was felt as far away as Madison, Wis., which is 134 miles away, according to the USGS.

The quake was centered about 1 mile south-southeast of Pingree Grove. The USGS initially had placed the epicenter 5 miles east of Sycamore and 4 miles west of Virgil.

Barb Huck of Crystal Lake said she was sleeping on the couch when she felt the shaking.

"My husband was upstairs and I thought he fell or something," she said.

Huck was the only one on the main floor of the house, and neither her husband or two sons felt the quake. There didn't appear to be any damage to her house.

"It's bizarre," she said. "It was a jolt."

Two pictures fell from the wall in Fred Alopogianis' West Dundee house. But his first thought was that the roof of his house was caving from the recent heavy snowfall.

"I said, 'Wait a minute, the roof is pitched and the house is only 6 years old,' " he said. "After that, my dog started barking because he thought somebody was trying to break in," Alopogianis said.

Alopogianis looked out the windows, thinking a car might have struck the house, but found out what the shaking actually was by turning on the news.

It wasn't the first earthquake he has felt, although he didn't expect to feel one in Illinois, Alopogianis said. Twice on visits to California he felt them, including one while standing on the 10th floor of a hotel and saw the water in a pool splashing.

The USGS originally said the quake was 4.3 in magnitude before downgrading it just before 8 a.m. Wednesday. Paul Stoddard, an associate professor in the department of Geology and Environmental Geosciences at Northern Illinois University, said the numbers are fluid during the first couple of hours after an earthquake.

John Bellini, a geophysicist with the National Earthquake Information Center, said the numbers are being revised as more data become available. The same is true as to the location of the epicenter, he said.

There is no known fault line in this area, Stoddard said, but there "is a fault there because there was an earthquake. Anywhere on the planet can see an earthquake. Some places get them a lot more frequently."

There have been a handful of earthquakes in northern Illinois during the past decade, he said, including one in 2008 that was south of DeKalb County and was registered at a 5.1 magnitude.

Earthquakes, generally speaking, are caused by a shifting of rocks, Stoddard said. In the Midwest, it's most likely due to what is called glacial rebound.

"We used to have huge glaciers sitting over Great Lakes region and into Canada," he said. "When they melted, that releases a lot of weight from that part of the world. The surface of the Earth rebounds upward. Imagine taking a bowling ball off a mattress; only with the earth it happens a lot more slowly. As the surface rises up, small adjustments in the rock happen occasionally. And we felt one this morning."

There were no reports of damages. Stoddard said a quake this small would result in little to no damage.

Kane County sheriff's spokesman Lt. Pat Gengler said dispatchers have been flooded with calls from startled residents, though no injuries or damage have been reported. He said earthquakes in the area were rare.

Residents reported being shaken out of bed and finding books and tools scattered across the floor after falling from shelves.

USGS geophysicist Amy Vaughan said such quakes were rare in northern Illinois. She said the agency received reports from Wisconsin, Iowa and Indiana about feeling the ground shake.

• The Associated Press contributed to this report.

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